1. Field of the Invention
The present invention relates generally to aircraft control systems and more particularly to a system for the achievement and maintenance of a desired cruise airspeed for the aircraft by a coordinated control and operation of the aircraft's engine throttles and its pitch attitude.
2. Description of the Prior Art
The problem of achieving and maintaining a cruise speed which results in minimizing the fuel consumption on modern jet aircraft is well known and is discussed further in Applicants' assignee's copending application Ser. No. 332,901, now U.S. Pat. No. 4,490,793, entitled "Cruise Speed Control for Aircraft Performance Management System" as filed on Dec. 21, 1981. As discussed therein, the basic difficulty is that the optimal cruise speed typically lies in a region of almost neutral speed stability; that is, large deviations in desired Mach number can occur at virtually constant engine thrust or engine pressure ratio (EPR) settings, and conversely, very small changes in engine thrust or EPR may result in very large Mach number changes.
Many prior art automatic throttle control systems used only a signal proportional to the difference between the desired optimal Mach number speed and the actual Mach number speed to alter the thrust setting of the engine(s). However, because of the neutral speed stability phenomenon, such systems exhibited oscillatory speed and throttle control characteristics that adversely affected fuel consumption and engine life and at the same time were objectionable to passengers and crew.
There have been recent attempts in the prior art to overcome the virtual neutral speed stability problem at aircraft cruise speeds. In one attempt, the algebraic sum of Mach speed error, that is, the difference between the commanded Mach number speed and the actual Mach number speed, the integral of Mach error, and the rate of change of actual Mach number was used to drive the aircraft's autothrottle system to adjust the thrust or EPR of the engines. In addition, a signal based on actual Mach rate, or alternatively, washed out actual Mach number was supplied to the pitch channel of the aircraft's autopilot where it was summed with the altitude hold signal when the autopilot was operating in its altitude hold mode in an attempt to reduce any detected Mach rate to zero. A turbulence detector was also used to override both the throttle control signal and the pitch cross control signal when the aircraft experienced unsteady atmospheric disturbances. While some success may have been achieved with this system, it had at least three basic shortcomings. In the first instance, the reduction of Mach rate to zero through the pitch autopilot does not ensure that Mach error is reduced to zero as is desired, it only ensures that the aircraft is not accelerating or decelerating. Thus, significant Mach errors due to relatively rapid atmospheric changes can be maintained until the autothrottles can adjust the EPR to correct for the error. In the second instance, there exists no dynamic separation between the pitch autopilot commands and the autothrottle commands. Any dynamic changes in actual Mach number due to pitching the aircraft to reduce the Mach rate signal to zero are directly reflected into the autothrottle system, resulting in unnecessary and undesirable throttle activity. In the third instance, the prior art example has no provision for the timely acquisition of the desired Mach number should the autothrottle system be engaged at a speed significantly different from the desired speed.
Another prior art approach to the solution of the neutral speed stability problem is described in the above referenced copending application, U.S. Ser. No. 332,901, now U.S. Pat. No. 4,490,793. In this system, a bias signal is introduced into the pitch autopilot altitude hold loop that tends to displace the aircraft from its reference altitude by an amount proportional to the detected Mach error. The resulting altitude error, .DELTA. H, defined as the difference between the reference altitude and the actual altitude, is used as the input to an integrator in the autothrottle control loop, the output of which is used to adjust the engine pressure ratio or thrust. While this system is generally satisfactory, one of its drawbacks is that the altitude error .DELTA. H, input to the autothrottle is affected by factors other than the proportional altitude hold bias generated by Mach error. Atmospheric disturbances, such as gusts and generally long term pressure waves, and the initial loss of altitude in turns can also affect this .DELTA. H term, resulting in unnecessary throttle activity and speed excursions. In addition, no provision is made for the timely acquisition of the desired Mach number should the system be engaged at a Mach number significantly different from the desired Mach number.
The present invention differs from the first of the above attempts to solve the neutral speed stability problem in that the primary pitch control into the aircraft's autopilot is Mach error instead of Mach rate or washed out Mach error, and that the primary input to the autothrottle system is Mach error which has been passed through a special isolation filter and summed with the integral of Mach error. This combination not only eliminates the need for a turbulence detector, but more importantly serves to isolate the dynamics of the automatic pilot pitch axis control system from the dynamics of the autothrottle control system, thus minimizing or eliminating any cross coupling effects inherent in the prior art system first described above. In addition, provisions are made through an independent control to allow for timely acquisition of the desired Mach number.
The present invention is a significant improvement of the system described in U.S. Ser. No. 332,901 in that the primary input to the autothrottle system is filtered Mach error plus integrated Mach error instead of integrated altitude error. The control system of the present invention eliminates the adverse effects of atmospheric disturbances and aircraft bank angles on the autothrottle system, thereby desirably minimizing throttle activity. In addition, an independent control is used which allows for the timely acquisition of the desired Mach number.